Lead-acid batteries of the tubular type are customarily constructed with positive plates, constituent parts of which include: a grid member having a top bar; a post or burning lug and a plurality of current carrying spines in spaced relation depending from one side of the top bar; pencils of active material surrounding the current carrying spines; tubular bodies arranged to support and confine the pencils of active material and maintain such active material in contact with the current carrying spines; and means for closing ends of the tubes.
The tubular bodies are required to provide a number of functions including: enclosure and support of the active material to maintain said active material in contact with the spines; maintenance of the dimensional stability of the pencils of active material particularly during periods of swelling of the said active material; and, finally, provision of adequate communication between the electrolyte and the active material throughout the length of the tubular bodies.
"Porosity" is the term commonly employed to define the degree of communication between the active material and the electrolyte through the wall portions of the tubular bodies. Porosity can be further defined mathematically as the ratio of the surface area of openings in the wall portions to the overall surface area of the tubular bodies, commonly expressed in terms of a percentage.
In batteries which are intended for relatively high discharge rates the porosity of the tubular bodies may be relatively high, e.g. in the order of 50 to 70 percent. In batteries intended for low discharge rates porosity may be relatively lower, e.g. in the order of 30 to 50 percent.
With reference to the aforementioned functions of a tubular body, variables in the design of the tubular body of the class noted include: "strength", which is a function of the material selected and of the porosity; "volumetric displacement of electrolyte", which is a function of wall thickness and which will be increased if an overlapping seam is present within the wall; "porosity", previously defined and which may decrease should an overlapping seam be present in the tube wall if the pores in the overlapping portions do not precisely overlie one another; undesirable "gas polarization", i.e. impedance to the passage of gas bubbles which evolve during charging and which is related to the configuration of the pores, being greatest when the pores are round openings.
Obviously, an optimized tubular body will incorporate some compromise between the optimum values of these variables. The value of certain of the variables, e.g. porosity, strength, configuration etc., may be dictated by optimal design of the battery cell with respect to the use for which it is intended.
The prior art, recognizing these facts, has evolved four general types of tubes presently in use. The first is a double-wall structure having an inner seamless tube braided from staple fibers, usually glass or a mixture of glass and plastic, over which is formed an outer strength tube of lapped and seamed construction made from perforated polyvinyl choride sheet material. This type of tube provides certain disadvantages, for example: the wall thickness is relatively great; a lapped seam is present which increases volumetric displacement of electrolyte; the perforations or openings in the lapped portions may not overlie one another, thus reducing porosity at these portions; cost of perforation dies is extremely high so that changes in porosity are prohibitively expensive; and polyvinyl chloride may not be chemically inert in the electrolyte and thus may release harmful chlorides into the battery cell. The porosity of this type of tube is commonly expressed as the ratio between the area of the perforation of the sheet material to the total area of the tube in terms of a percentage.
The second type of tube is a seamless single-wall construction again braided from staple fibers but then impregnated with a stabilizing material such as a phenolic resin. This type of tube is less strong and less dimensionally stable than the first type, and the phenolic resin tends to break down during operation of the battery leaving the tube with little dimensional stability and this breakdown may introduce potentially harmful organic compounds such as acetic acid into the electrolyte. In some cases axially disposed fibers may be included in the braid.
The third type is a multiple tube array commonly woven from staple fibers on a cartridge-belt type loom. The structure is customarily impregnated with a stabilizing resin of a nature such as that noted above and which is subject to the same disadvantages.
The fourth type is also a multiple tube array formed from non-woven foraminous material, e.g. a needle-punched polyester, sheets of which are stitched or sewed together to form individual tubes.
All of these present configurations present compromises between the various cited requirements for an optimal tubular body. Changes in precise control of porosity and cross-sectional configuration are difficult or expensive to obtain in all cases, strength is compromised by mechanical design considerations, and some form of potential chemical contamination is generally present. In addition, such tubes are customarily made, shipped and stored in a generally cylindrical shape or other shape necessarily requiring a great deal of space which increases the cost of said shipment and storage.